Structural,optical and dielectric properties of Ni substituted NdFeO3

Present study reports the structural, optical and dielectric properties of Ni substituted NdFe_1−xNi_xO₃ (0 ≤ x ≤ 0.5) compounds prepared through the ceramic method. X-ray diffraction (XRD) confirmed an orthorhombic crystal structure of all the samples. Both unit cell volume and grain size were found to decrease with an increase in Ni concentration. Morphological study by Scanning electron microscope (SEM) shows less porosity with Ni substitution in present system. From UV–vis spectroscopy, the optical band gap was found to increase with Ni doping. This observed behavior was explained on the basis of reduction in crystallite size, unit cell volume and its impact on the crystal field potential of the system after Ni substitution. The dielectric properties (?′ and tanδ) as a function of frequency or temperature, and the ac electrical conductivity (σ_ac) as a function of frequency have been studied. Hopping of charge carriers between Fe²⁺ → Fe³⁺ ions and Ni²⁺ → Ni³⁺ ions are held responsible for both electrical and dielectric dispersion in the system. Wide optical band gap and a very high dielectric constant of these materials promote them to be a suitable candidate for memory based devices in electronic industry.     

Mossbauer, infrared and magnetic susceptibility studies of iron sodium borate glasses doped by sulphur

The affect of sulphur on the structural properties of iron sodium diborate glasses having the composition {(100−x)Na₂B₄O₇+xFe₂O₃}+yS, where x=0.05, 0.15 and 0.25 mol% and Y=0, 2.5 and 5 wt% was studied by infrared, Mossbauer spectroscopy and magnetic susceptibility measurements. It was found that, for samples having 5 mol% Fe₂O₃ and free from sulphur, the iron ions are present in both Fe²⁺ and Fe³⁺ states and also 92% of the total iron enters the glass network as a glass former. The ratio of Fe³⁺/Fe²⁺ increases with increasing the iron content for sulphur-free samples and others containing sulphur. This ratio also decreases with increasing the sulphur content. The magnetic susceptibility was found to decrease with increasing the sulphur content. Also, the increase of Fe₂O₃ content led to a less symmetrical environment of Fe³⁺ ions and vice versa for the Fe²⁺ environment.     

Microstructure characterization and magnetic behavior of NiAlxFe2−xO4 and Ni1−yMnyAl0.2Fe1.8O4 ferrites

NiAl_xFe_2−xO₄ and Ni_1−yMn_yAl_0.2Fe_1.8O₄ ferrites were prepared by the conventional ceramic method and were characterized by X-ray diffraction, scanning electron microscopy, and magnetic measurements. The single spinel phase was confirmed for all prepared samples. A proper explanation of data is possible if the Al³⁺ ions are assumed to replace Fe³⁺ ions in the A and B sites simultaneously for NiAl_xFe_2−xO₄ ferrites, and if the Mn²⁺ ions are assumed to replace Ni²⁺ ions in the B sites for Ni_1−yMn_yAl_0.2Fe_1.8O₄ ferrites. Microstructural factors play an important role in the magnetic behavior of Ni_1−yMn_yAl_0.2Fe_1.8O₄ ferrites with large Mn²⁺ content.     

Structural and luminescence characterization of synthetic Cr-doped Ni3(PO4)2

Ni_3–xCr_2x/3(PO₄)₂ (x=0 and 0.02) microcrystalline powders were obtained as single phases via a modified sol–gel Pechini-type in situ polymerizable complex method. The samples were characterized using scanning electron microscopy, X-ray diffraction, cathodoluminescence (CL), and thermoluminescence (TL) techniques. We found that Cr³⁺ doping modified the average particle and distribution. The mean particle size was 0.441 μm for Ni₃(PO₄)₂ and 0.267 μm for Ni_2.98Cr_0.013(PO₄)₂. The results also reveal that Cr³⁺ doping notably enhanced the CL and TL UV-blue emission.     

Structural studies of Cr-doped mullite derived from single-phase precursors

Cr-doped mullites were prepared from single-phase precursors containing up to 9.60 wt% Cr₂O₃ using a sol-gel technique followed by thermal treatment. Particle induced X-ray emission spectroscopy and X-ray powder diffraction were used to characterize the samples. Mullites were orthorhombic, space group Pbam. Cr doping caused the increase of unit-cell parameters. Strongest expansion was noticed along c-axis followed by a and b (Δc/c=0.089, Δa/a=0.061, Δb/b=0.045% per mole Cr₂O₃). A second phase, namely θ-(Al,Cr)₂O₃, was revealed by XRD in the sample containing 9.60 wt% Cr₂O₃. The structure of mullites was refined by the Rietveld method, location of Cr³⁺ was performed by the EPR spectroscopy. At low chromium doping level (Cr₂O₃ content less than ∼5 wt%) Cr³⁺ ions were substituted for Al³⁺ in the AlO₆ octahedra of the mullite structure (M1 site). For higher doping level, Cr³⁺ ions were additionally substituted for Al³⁺ in the AlO₆ octahedra of the second phase [θ-(Al,Cr)₂O₃ at 1400 °C, or α-(Al,Cr)₂O₃ at 1600 °C] which segregated in the system. Substitution of Cr³⁺ for Al³⁺ on M1 site in the mullite structure resulted in increase of average distances in (M1)O₆ octahedron and decrease of average distances in T*O₄ tetrahedron, while average distances in TO₄ tetrahedron stayed almost constant.     

Structural, thermal and magnetic properties of Ni1−xMnxFe2O4 nanoferrites

In this paper, the structural, thermal and magnetic properties of Ni_1−xMn_xFe₂O₄ are presented. It is observed that high concentration of Mn²⁺ ions into NiFe₂O₄ tends to reduce the particle size. Calcination at 500 °C has resulted in the growth of Ni_1−xMn_xFe₂O₄ nanoparticles, but the calcination at 900 °C has led to the evaporation of the majorities of the polyvinyl alcohol. After calcination at 900 °C, crystallographically oriented NiMnFe₂O₄ nanoparticles are formed. These Ni_1−xMn_xFe₂O₄ nanoparticles show hysteresis behaviour upon magnetization. On the other hand, saturation magnetization (Ms) values decreases with increasing Mn content in ferrite due to the influence of Mn²⁺ ion in the sub lattice.     

Antiferromagnetism and spin-glass transition in the FeInxCr2−xSe4 series of selenides

The magnetic behavior of the FeIn_xCr_2−xSe₄ system (with x=0.0, 0.2 and 0.4) has been investigated by magnetic and Mössbauer spectroscopy. Hyperfine parameters indicate that iron is in the Fe²⁺ oxidation state, with a minor (∼9%) Fe³⁺ fraction, located at different layers in the structure. Low-field magnetization curves as a function of temperature showed that the antiferromagnetic (AFM) order temperature is T_N=208(2) K for FeCr₂Se₄ and decreases to 174(3) K for FeIn_0.4Cr_1.6Se₄. The effective magnetic moment μ_eff decreases with increasing In contents, and shows agreement with the expected values from the contribution of Fe²⁺ (⁵D) and Cr³⁺ (⁴F) electronic states. A second, low-temperature transition is observed at T_G∼13 K, which has been assigned to the onset of a glassy state.     

Chemical synthesis of spinel nickel ferrite (NiFe2O4) nano-sheets

The present investigation is related to the deposition of single-phase nano-sheets spinel nickel ferrite (NiFe₂O₄) thin films onto glass substrates using a chemical method. Nano-sheets nickel ferrite films were deposited from an alkaline bath containing Ni²⁺ and Fe²⁺ ions. The films were characterized for their structural, surface morphological and electrical properties by means of X-ray diffraction (XRD), transmission electron microscope (TEM), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), and two-point probe electrical resistivity techniques. The X-ray diffraction pattern showed that NiFe₂O₄ nano-sheets are oriented along (3 1 1) plane. The FT-IR spectra of NiFe₂O₄ films showed strong absorption peaks around 600 and 400 cm⁻¹ which are typical for cubic spinel crystal structure. Microstructural study of NiFe₂O₄ film revealed nano-sheet like morphology with average sheet thickness of 30 nm. The room temperature electrical resistivity of the NiFe₂O₄ nano-sheets was 10⁷ Ω cm.     

High-temperature electrical properties of magnesiowustite Mg1 − xFexO and spinel Fe3 − x − yMgxCryO4 ceramics

Phase relationships, thermal expansion and electrical properties of Mg_1 − xFe_xO (x = 0.1-0.45) cubic solid solutions and Fe_{3 − x − y}Mg_xCr_yO_4 ± δ (x = 0.7-0.95; y = 0 or 0.5) spinels were studied at 300-1770 K in the oxygen partial pressure range from 10 Pa to 21 kPa. Increasing iron content enlarges the spinel phase stability domain at reduced oxygen pressures and elevated temperatures. The total conductivity of the spinel ceramics is predominantly n-type electronic and is essentially p(O₂)-independent within the stability domain. The computer simulations using molecular dynamics technique confirmed that overall level of ion diffusion remains low even at high temperatures close to the melting point. Temperature dependencies of the total conductivity in air exhibit a complex behavior associated with changing the dominant defect-chemistry mechanism from prevailing formation of the interstitial cations above 1370-1470 K to the generation of cation vacancies at lower temperatures, and with kinetically frozen cation redistribution in spinel lattice below 700-800 K. The average thermal expansion coefficients of the spinel ceramics calculated from dilatometric data in air vary in the range (9.6-10.0) × 10^− 6 K^− 1 at 300-500 K and (13.2-16.1) × 10^− 6 K^− 1 at 1050-1370 K. Mg_1 − xFe_xO solid solutions undergo partial decomposition on heating under oxidizing and mildly reducing conditions, resulting in the segregation of spinel phase and conductivity decrease.     

Influence of the concentration in Fe-doped BaTiO3 on magnetoelectric couping of layered composites BaTi1−xFexO3–Tb1−yDyyFe2−z

Perovskites BaTi_1−xFe_xO₃ has been synthesized with the concentration x ranging from 0.01 to 0.02. Their transformation point of ferroelectric to paraelectric and the corresponding latent heat of the phase transformation were observed to decrease with increasing the doping level of Fe³⁺. Bonded layered composites BaTi_1−xFe_xO₃–Tb_1−yDy_yFe_2−z have been fabricated and their magnetoelectric effect has been investigated. The sample containing a layer of perovskite BaTi_0.985Fe_0.015O₃ was found to show the maximum transverse ME voltage coefficient, which is about 1422 mV Oe⁻¹ cm⁻¹ under a magnetic field of 1580 Oe, in these bilayers. Analysis shows that the Fe-doped BaTiO₃ with doping level at about 1.5% should have largest piezoelectric coefficient in these ceramics BaTi_1−xFe_xO₃.     

Single crystal EPR and optical absorption study of Cr doped l-histidine hydrochloride monohydrate

EPR study of the Cr³⁺ ion doped l-histidine hydrochloride monohydrate single crystal is done at room temperature. Two magnetically inequivalent interstitial sites are observed. The hyperfine structure for Cr⁵³ isotope is also obtained. The zero field and spin Hamiltonian parameters are evaluated from the resonance lines obtained at different angular rotations and the parameters are: D=(300±2)×10⁻⁴ cm⁻¹, E=(96±2)×10⁻⁴ cm⁻¹, g_x=1.9108±0.0002, g_y=1.9791±0.0002, g_z=2.0389±0.0002, A_x=(252±2)×10⁻⁴ cm⁻¹, A_y=(254±2)×10⁻⁴ cm⁻¹, A_z=(304±2)×10⁻⁴ cm⁻¹ for site I and D=(300±2)×10⁻⁴ cm⁻¹, E=(96±2)×10⁻⁴ cm⁻¹, g_x=1.8543±0.0002, g_y=1.9897±0.0002, g_z=2.0793±0.0002, A_x=(251±2)×10⁻⁴ cm⁻¹, A_y=(257±2)×10⁻⁴ cm⁻¹, A_z=(309±2)×10⁻⁴ cm⁻¹ for site II, respectively. The optical absorption studies of single crystals are also carried out at room temperature in the wavelength range 195-925 nm. Using EPR and optical data, different bonding parameters are calculated and the nature of bonding in the crystal is discussed. The values of Racah parameters (B and C), crystal field parameter (Dq) and nephelauxetic parameters (h and k) are: B=636, C=3123, Dq=2039 cm⁻¹, h=1.46 and k=0.21, respectively.     

Positive exchange bias in a Ni80Fe20/NixFe1−xO thin-film bilayer

We have measured positive exchange bias in a Ni₈₀Fe₂₀/Ni_xFe_1−xO thin-film nanocrystallite system. A series of solid solution Ni_xFe_1−xO 40 nm thick films capped with 25 nm thick Ni₈₀Fe₂₀ were deposited using a range of %O₂/Ar bombardment energies (i.e. End-Hall voltages). Proper tuning of the deposition conditions results in a Ni₈₀Fe₂₀/Ni_xFe_1−xO (30%O₂/Ar) based bilayer that exhibits a positive exchange bias loop shift of H_ex∼60 Oe at 150 K.     

Preparation and characterization of carbons for the retention of halogens in the condenser vacuum system of a thermonuclear plant

Activated carbons were prepared by air and carbon dioxide activation, from almond tree pruning, with the aim of obtaining carbons that reproduce the textural and mechanical properties of the carbons currently used in the filtering system of the condenser vacuum installation of a Thermonuclear Plant (CNA; Central Nuclear de Almaraz in Caceres, Spain), produced from coconut shell. The variables studied in non-catalytic gasification series with air were the temperature (215-270 °C) and the time (1-16 h) and the influence of the addition of one catalyst (Co) and the time (1-2 h) in catalytic gasification. In the case of activation with CO₂, the influence of the temperature (700-950 °C) and the time (1-8 h) was studied. The resulting carbons were characterized in terms of their BET surface, porosity, and pore size distribution. The N₂ adsorption isotherms at 77 K for both series showed a type I behaviour, typical of microporous materials. The isotherms showed that with both gasificant agents the temperature rise produced an increase in the carbon porosity. With regards to the activation time, a positive effect on the N₂ adsorbed volume on the carbons was observed. The best carbons of each series, as well as the CNA (carbon currently used in the CNA), were characterized by mercury porosimetry and iodine solution adsorption isotherms. The results obtained allowed to state that several of the carbons produced had characteristics similar to the carbon that is target of reproduction (which has S_BET of 741 m² g⁻¹, V_mi of 0.39 cm³ g⁻¹ and a iodine retention capacity of 429.3 mg g⁻¹): carbon C (gasification with CO₂ at 850 °C during 1 h), with S_BET of 523 m² g⁻¹, V_mi of 0.33 cm³ g⁻¹ and a iodine retention capacity of 402.5 mg g⁻¹, and carbon D (gasification with CO₂ at 900 °C during 1 h), whose S_BET is 672 m² g⁻¹, V_mi is 0.28 cm³ g⁻¹ and has a iodine retention capacity of 345.2 mg g⁻¹.     

Synthesis and magnetic properties of Au-coated amorphous Fe20Ni80 nanoparticles

Gold-coated nanoparticles of Fe₂₀Ni₈₀ (permalloy) have been synthesized by a microemulsion process. The as-prepared samples consist of ∼5 nm diameter particles of amorphous Fe₂₀Ni₈₀ that are likely encapsulated in B₂O₃. One or more Fe₂₀Ni₈₀@B₂O₃ particles are subsequently encapsulated in 8-20 nm gold nanospheres, as determined by TEM and X-ray powder diffraction (XRD) line broadening. The gold shells were found to be under expansive strain. Magnetic data confirm the existence of a superparamagnetic phase with a blocking temperature, T_B, of ∼33 K. The saturation magnetization, M_S, of the as-prepared, Au-coated sample is ∼65 emu g⁻¹ at 5 K and ∼16 emu g⁻¹ at 300 K. The coercivity, H_C, is ∼280 Oe at 5 K.     

2-Propanol photodegradation over nitrogen-doped NaNbO3 powders under visible-light irradiation

Nitrogen-doped perovskite-type materials, yellowish NaNbO_3−xN_x powders, had been developed as visible-light-sensitive photocatalysts for decomposition of gaseous 2-propanol. The NaNbO_3−xN_x samples were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), ultraviolet-visible (UV-vis) light diffuse reflectance spectroscopy, Brunauer-Emmett-Teller (BET) measurement, and scanning electron microscopy (SEM). The UV-vis spectra confirmed that the spectral response of the NaNbO₃ powders could be tuned to visible-light region by nitrogen doping technique. The photocatalytic activities of NaNbO_3−xN_x samples were evaluated by decomposing gaseous 2-propanol into acetone and CO₂ under visible-light irradiation (400 nm<λ<520 nm). The NaNbO_3−xN_x sample annealed at 833 K showed the highest visible-light photocatalytic activity among all the nitrogen-doped samples. The relationship between nitrogen doping amount and photocatalytic activity of NaNbO_3−xN_x samples was also investigated and discussed.     

Solid state reaction synthesis of NiFe2O4 nanoparticles

Ni-ferrite (NiFe₂O₄) nanoparticles have been synthesized via a solid state reaction process. Ni and Fe bi-metallic nanoparticles in the form of Ni₃₃Fe₆₇ alloy nanopowder are first synthesized by simultaneous evaporation of the required amounts of pure Ni and Fe metals followed by rapid condensation of the evaporated metal flux into solid state by means of an inert gas, helium, using the process of inert gas condensation (IGC). In order to form the NiFe₂O₄ structure, as-synthesized samples (Ni₃₃Fe₆₇) are annealed for 12 h in ambient conditions at different annealing temperatures. Structural analyses show that NiFe₂O₄ starts to form at around 450 °C and gets progressively well defined with increasing annealing temperatures yielding particle with size ranging between 15 and 50 nm. Besides successfully forming NiFe₂O₄, NiO/Fe₃O₄ core/shell nanoparticles have also been synthesized by adjusting the annealing conditions. Three different structures, Ni₃₃Fe₆₇, NiO/Fe₃O₄, and NiFe₂O₄, obtained in this study are compared with respect to their structural and magnetic properties.     

The synthesis and the magnetic properties of Gd-doped FexCo1−x/CoyFe3−yO4 micro-octahedrons composites

Gd³⁺-substituted micro-octahedron composites (Fe_xCo_1−x/Co_yGd_zFe_3−y−zO₄) in which the Fe-Co alloy has either a bcc or fcc structure and the oxide is a spinel phase were fabricated by the hydrothermal method. The X-ray diffraction (XRD) patterns indicate that the as-synthesized Gd³⁺-substituted micro-octahedron composites are well crystallized. Scanning electron microscopy (SEM) images show that the final product consists of larger numbers of micro-octahedrons with the size ranging from 1.3 to 5 μm, and the size of products are increased with increasing the concentration of KOH. The effect of the Co²⁺/Fe²⁺ ratio (0?Co²⁺/Fe²⁺?1) and substitution Fe³⁺ ions by Gd³⁺ ions on structure, magnetic properties of the micro-octahedrons composites were investigated, and a possible growth mechanism is suggested to explain the formation of micro-octahedrons composites. The magnetic properties of the structure show the maximal saturation magnetization (107 emu/g) and the maximal coercivity (1192 Oe) detected by a vibrating sample magnetometer.     

Advanced electrochemical performance of LiMn1.4Cr0.2Ni0.4O4 as 5 V cathode material by citric-acid-assisted method

Spinel LiMn₂O₄ and LiMn_1.4Cr_0.2Ni_0.4O₄ cathode materials were successfully synthesized by the citric-acid-assisted sol-gel method with ultrasonic irradiation stirring. The structure and electrochemical performance of the as-prepared powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) spectrometer, cyclic voltamogram (CV) and the galvanostatic charge-discharge test in detail. XRD shows that all the samples have high phase purity, and the powders are well crystallized. SEM exhibits that LiMn_1.4Cr_0.2Ni_0.4O₄ has more uniform cubic-structure morphology than that of LiMn₂O₄. EDX reveals that a small amount of Mn³⁺ still exists in LiMn_1.4Cr_0.2Ni_0.4O₄. The galvanostatic charge-discharge test indicates that the initial discharge capacities for the LiMn_1.4Cr_0.2Ni_0.4O₄ and LiMn₂O₄ at 0.15 C discharge rates are 130.8 and 130.2 mAh g⁻¹, respectively. After 50 cycles, their capacity are 94.1% and 85.1%, respectively. The CV curve implies that Ni and Cr dual substitutions are beneficial to the reversible intercalation and deintercalation of Li⁺, and suppress Mn³⁺ generation at high temperatures and provide improved structural stability.     

Graphite nanosheets doped with Fe, Ni, and N, synthesized in one step, and their unique magnetic performance

Graphite nanosheets (GNs) doped with N, Fe, or Ni were synthesized by pyrolysis of metal tetrapyridinoporphyrazine (MPTpz, M=Fe²⁺, and Ni²⁺) and a mixture of MPTpzs in a chemical vapor deposition furnace. The products obtained were characterized by scanning and transmission electron microscopy, and X-ray photoelectron spectroscopy. The magnetic properties of the GNs obtained were investigated at room temperature using a vibrating sample magnetometer with an applied field of −10 000-10 000 Gs. The results show the GNs obtained are terrace-like and ultra-thin, with very high aspect ratio. Fe, Ni and N atoms have been doped to the GNs successfully. There are two types of N atom that are introduced into pure carbon systems: pyrinidic and graphitic N atoms. The GNs obtained exhibit ferromagnetic behavior at room temperature. Sample S1, obtained by pyrolysis of a mixture of MPTpzs (M=Fe²⁺ and Ni²⁺), have the highest coercivity force. The saturation magnetization (M_s), remanent magnetization (M_r), and coercivity (H_c) values of sample S1 are 24.51 emu g⁻¹, 3.95 emu g⁻¹, and 207.34 Gs, respectively.     

Fabrication and characterization of electrodeposited Co1−xCrx nanowires

Co_1−xCr_x   alloy nanowires with 0.01<x<0.93$0.01<x<0.93$ were fabricated by electrodeposition in a porous alumina membrane from an electrolyte containing Co and Cr ions. The composition, structure and magnetic properties of the nanowires have been characterized. Cobalt-rich nanowires were electrodeposited at a potential of −1.0 V relative to Ag/AgCl and chromium-rich nanowires were deposited beyond −3.5 V. The optimized processing conditions include hydrogen annealing to give hysteresis loops for the Co₈₀Cr₂₀ nanowires with coercivity of up to 200 mT and squareness of up to 0.95. Magnetization of the Co₈₀Cr₂₀ nanowire is 77 A m² kg⁻¹ and the energy product of the arrays is 35 kJ m⁻³.